The instant invention relates to filters for the removal of particulate material from diesel engine exhaust, and more particularly to a method of testing the integrity of diesel particulate filters (DPFs) for detecting defects which affect the filtration efficiency.
Wall-flow diesel particulate filters are used in the purification of diesel exhaust. Typically such diesel particulate filters are made of cordierite (U.S. Pat. No. 4,420,316) or silicon carbide (U.S. Pat. No. 5,914,187), and include a honeycomb body having thin interconnecting interior porous walls which form parallel cell channels of equal hydraulic diameter, longitudinally extending between the end faces of the structure. Alternating cells on one end face of the honeycomb are plugged with a ceramic filler material to form a xe2x80x9ccheckerboardxe2x80x9d pattern. The pattern is reversed on the opposite side, so that the ends of each cell are blocked at only one end of the structure. When diesel exhaust enters the filter through one end face (i.e., inlet end), it is forced to pass through the porous walls in order to exit through the opposite end face (i.e., outlet end).
For diesel particulate filtration, honeycomb structures having cellular densities between about 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2), are considered useful to provide sufficient thin wall surface area in a compact structure. Wall thickness can vary upwards from the minimum dimension providing structural integrity of about 0.002 in. (about 0.05 mm.), but are generally less than about 0.060 in. (1.5 mm.) to minimize filter volume. A range of between about 0.010 and 0.030 inches (about 0.25 and 0.76 mm.) e.g., 0.019 inches, is most often selected for these materials at the preferred cellular densities.
Filtration efficiencies up to and in excess of 90% of the diesel exhaust particulates (by weight) can be achieved with the described structures. However, the filtration efficiency or integrity of DPFs can be compromised by defects from the manufacturing process, such as breaks or holes in the interior cell walls. Such defects allow diesel exhaust containing soot particulates to pass through the structure with no filtration.
Methods of inspecting internal defects affecting described diesel filters are known in the art. U.S. Pat. No. 5,102,434 to Hijikata et al. is directed to a method in which a gas containing particulates, such as carbon soot, is flowed through one end of the filter, exiting the opposing end adjacent a gas-permeable screen. The carbon soot or other particulates employed will escape through leaks or defects in the interior walls to adjacent open cells eventually being captured onto the screen. The screen is then inspected for patterns differing from the defect-free structure.
In another method a clear film is taped to one end of the honeycomb structure and graphite is then blown through the opposite end. The structure is rotated several times. Leaking cells are detected by the presence of graphite on the clear film.
There are several disadvantages associated with these prior art methods. Particularly, both methods required a fired plugged part. It is more costly to repair defect cell channels in the final product, then, for example, while the structure is still green (i.e., has not been fired). Further, in some instances leaking cells are never detected due to what is believed to be bridging of the particulates (i.e., particulates are too big to flow through small cracks). Therefore, there continues to be a need for more reliable and efficient methods of testing the integrity of diesel particulate filters.
The present invention overcomes the above problems and drawbacks by providing a more reliable and efficient method for detecting the presence of integrity leakage defects in ceramic filters for trapping and combusting diesel exhaust particulates. The term xe2x80x9cintegrity leakage defectsxe2x80x9d used in the description of the present invention refers to any breaks, tears, holes and the like in the interior walls or at joining sections of the filter body.
In particular, the invention provides a method for testing comprising providing a green plugged honeycomb structure having a plurality of parallel cell channels traversing the structure from an inlet end to an outlet end thereof; passing air under pressure through the inlet end of the honeycomb structure; and detecting airflow at the outlet end of the honeycomb structure, whereby the presence of the air at the outlet end indicates integrity leakage defects in the ceramic filter. The term xe2x80x9cgreen filterxe2x80x9d as used in the description of the present invention refers to a structure which has not undergone firing. A green filter has no porosity such that the air cannot pass through the internal walls, unless there is an integrity leakage defect thereat.
The invention is also related to an assembly for testing the integrity of diesel exhaust filters, comprising an enclosure having an aperture; a green plugged filter removably mounted in the aperture of the enclosure; an air blower in communication with the enclosure; and means for detecting air leakage in the green plugged filter.
Therefore, in the present inventive method, in the case where there are no integrity leakage defects in the filter body, airflow will not be present at the end surface opposing the end where air is introduced in the structure. Conversely, in the case where there are integrity leakage defects in the filter body, airflow will be present at the end surface opposing the end where air is introduced in the structure. This is because the air will pass through the defects into adjoining open cell channels to flow out of the structure. By detecting the presence of airflow at an end surface, the number and location of integrity leakage defects in the filter body is determined.